1. Field of the Invention
The present invention relates to an automatic sampler for injecting a sample through a sampling needle into an analysis flow path of an analyzer such as a liquid chromatograph, and a method for injecting a sample with the use of such an automatic sampler.
2. Description of the Related Art
In an automatic sampler for a liquid chromatograph, a sampling needle is moved down toward a sample container so as to penetrate a septum hermetically sealing the sample container. Then, a predetermined amount of a sample is sampled from the sample container by sucking the sample through the tip of the sampling needle, and the sample is injected into an analysis flow path of the liquid chromatograph (see Japanese Patent Application Laid-open No. 2003-215118).
A sample container is usually sealed with a highly airtight septum to prevent the volatilization of a sample and the entry of foreign matter. Such a septum is made of a material through which a sampling needle can penetrate. At the time of sample suction, a sampling needle is allowed to penetrate a septum so that the tip of the sampling needle is inserted into a sample container, and then a sample is sucked through the sampling needle.
A conventional method for sucking a sample through a sampling needle will be described with reference to FIG. 7. The top of a sample container 3 placed in a rack is covered with a septum 26 made of an aluminum seal and the like to prevent the vaporization of a sample 24 contained in the sample container 3 and the entry of foreign matter from outside. A sampling needle 5 is moved to a position above the sample container 3 to suck the sample 24 (see (a)). Then, the sampling needle 5 is moved down so as to penetrate the septum 26 (see (b)), and is then further inserted into the sample container 3 to suck the sample 24 (see (c) and 7(d)).
As shown in FIG. 7(b), the septum 26 is largely bent in a direction in which the sampling needle 5 is inserted into the sample container 3 (i.e., in a downward direction) in the course of penetration of the sampling needle 5 through the septum 26, and is then returned to its initial state at the moment of complete penetration of the sampling needle 5 through the septum 26. Due to such deformation of the septum 26, there is a case where air pressure in the sample container 3 is changed so that air enters a sample suction port of the sampling needle 5. If air enters the sample suction port of the sampling needle 5, the amount of the sample to be sucked is reduced by the amount of air. In this case, there is a problem that the amount of the sample to be sucked is smaller than a predetermined amount so that a peak area value obtained as a measurement value becomes smaller.
As one measure to solve such a problem, a sampling needle having a groove formed on the outer circumferential surface thereof has been used. In this case, even when the sampling needle penetrates a septum, air can enter and exit a sample container through the groove located between the sampling needle and the septum. However, it is difficult to polish such a grooved sampling needle, and therefore the surface roughness of the groove portion of the sampling needle is greater than that of the outer circumferential surface of the sampling needle. For this reason, the groove portion is poor in liquid repellency, and therefore if a sample is adhered to the groove portion, the sample is likely to remain there. Therefore, the grooved sampling needle is inferior to a sampling needle having a simple cylindrical shape in carry-over performance. Further, in the use of the grooved sampling needle, there is a problem that contamination is caused by a sample remaining in the groove portion.